So Open-hearth Process, separately removed The temperature must be exceedingly high in order to preserve the iron in its fluid state after the expulsion of the carbon; the entire absence of the latter is discovered by the application of the spectroscope, this being the most practical use of that most wonderful Instrument. The next operation is the adding of so much carbon as is needed to produce the steel re- quired, and this is done by putting into the converter a measured amount of very pure cast iron called Spiegtleism* and mixing it well with the metal by re-applying the blast for a short time. The now converted steel is transferred to the ladle B, which is swung round by the crane c, and the metal poured into the ingot through the hole D on releasing the plug at the bottom of the ladle. The ingots may be afterwards piled and rolled as previously described, to produce a fibrous steel, and if used for forging and welding purposes should not have too much carbon in their com- position; or, if required for steel castings, may be re-melted in suitable quantities, much as in the way already mentioned for cast iron. The Siemens-Martin, or open-hearth process, is carried on in a special kind of furnace, called a regenerative furnace, invented by Sir W. Siemens. Fig. 91 is a drawing which will shew all the necessary parts. A is the hearth, sloped in the side elevation, so that the metal may run out when tapped at T. A current of air is allowed to pass under the hearth at c, to prevent the melting of the iire-clay. The combustion of a mixture of common coal gas and air is the source of heat, the arrows showing the passage to the interior of the gas through the valve o, while the air enters through the valve A. In the figure the mixture is seen entering the right side of the furnace. Being ignited at j by means of a red-hot bar, gradually and carefully at first, the flame is directed by the roof on to the metal, and the heat passes away by the left side of the furnace, returning through the valves and past the damper D to the chimney. Were it not, however, for Siemens' beautiful regenerative principle a great deal of heat would be wasted. The regenerators are shewn at nurr; they are hajrd fire-clay or silica bricks piled as a grating. The rejected heat from the hearth is intercepted by those marked n R, so that * See Appendix II, j>. 790.